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Environmental fate & pathways

Adsorption / desorption

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Description of key information

For the following reasons, no determination was possible by Method C19 of Commission Directive 2001/59/EC (which constitutes Annex V of Council Directive 67/548/EEC).

The method guideline states that the measurement of adsorption coefficient should be carried out on substances in their ionised and unionised forms.

The test material is a salt and is made up of two components, abacavir and glutaric acid.

The abacavir component has predicted dissociation constants of 14.86, 6.53 and -3.38 using ACD/pKa 8.03.

The glutaric acid component has an experimental database match for dissociation constants of 5.50 and 4.39 using ACD/pKa DB 8.02.

Based on this information, this suggests that the abacavir component would be unionised between approximately 7.5 and 13.5. The glutaric acid component is unionised below approximately 3.5. As the pH of sewage treatment plants operate between pH 5.5 and 7.5 and the environmentally relevant range is between approximately 5.5 and 8.5, testing is usually conducted in the latter range. However, the test material is a salt containing amine groups. Experience has shown that positively charged nitrogens can interact with the column stationary phase by forces other than partitioning.

Key value for chemical safety assessment

Additional information

For the following reasons, no determination was possible by Method C19 of Commission Directive 2001/59/EC (which constitutes Annex V of Council Directive 67/548/EEC).

The method guideline states that the measurement of adsorption coefficient should be carried out on substances in their ionised and unionised forms.

The test material is a salt and is made up of two components, abacavir and glutaric acid.

The abacavir component has predicted dissociation constants of 14.86, 6.53 and -3.38 using ACD/pKa 8.03.

The glutaric acid component has an experimental database match for dissociation constants of 5.50 and 4.39 using ACD/pKa DB 8.02.

Based on this information, this suggests that the abacavir component would be unionised between approximately 7.5 and 13.5. The glutaric acid component is unionised below approximately 3.5. As the pH of sewage treatment plants operate between pH 5.5 and 7.5 and the environmentally relevant range is between approximately 5.5 and 8.5, testing is usually conducted in the latter range. However, the test material is a salt containing amine groups. Experience has shown that positively charged nitrogens can interact with the column stationary phase by forces other than partitioning.

Determinations were performed in order to obtain evidence which supported the unsuitability of the HPLC method. The method used was to predominately analyse the abacavir component within the test material in the presence of glutaric acid. It is probable that using the HPLC conditions specified in the guidelines, the glutaric acid component would not be detected at the wavelength selected. Adjusting the pH of the mobile phase to approximately 8.5 would suggest that testing would be performed with the abacavir component in its unionised form, whereas adjusting the pH to approximately 5 would ionise the amine groups within the test material. No alteration to the ionisation of the glutaric acid component would occur between the two pHs. No difference in the retention time was observed between the two pHs. Adjusting the pH of the mobile phase to approximately 3 had no further effect on the ionisation of the abacavir component from the pH5 determination. However, the elution time was considerably reduced to the dead-time of the column. At this pH, and on condition that it could be detected, the acid component should also be retained. The difference in the results was concluded to be due to test material interaction with the stationary phase by forces other than partitioning. At pH 3, the free silanols on the column were saturated with acidic hydrogens leaving only a small proportion of ionic bonding to occur. Hence, the test material eluted quickly. However, at pH 5, the stationary phase had significantly more free silanols available for interaction hence the significantly increased retention. Quantitative Structure Activity Relationships (QSAR'S) detailed in the Technical Guidance Document (TGD) can be unreliable and therefore were not used in the estimation of adsorption coefficient. Although correction factors can be applied, it has been shown that QSAR models can significantly under estimate the adsorption coefficient of ionic substances. Therefore, due to the unsuitability of the HPLC method and the QSAR assessment, and estimation was performed using specialist chemical estimation software.

Using fragment constant methodology, the log10 Koc value for the test of the test material has been estimated. The results are shown in the following table:

Component

PCKOCWIN Log10Koc

Abacavir

1.80

Glutaric Acid

1.07